March 2012
Intermediate to advanced
244 pages
4h 42m
Chinese
Content preview from JavaScript 設計模式
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148
第七章
儲存在 Closure 中的實體
另一種作法是使用 closure 來保護那份唯一實體。你可以藉由第五章討論的 private 靜態成
員模式來實作。這裡的秘訣是重新定義建構式：
原本的建構式在第一次會被呼叫，而它也如常回傳
this
。接著第二次以後，執行的都是
重新定義的建構式。新的建構式會透過 closure 取得 private 的
instance
並回傳。
這份實作其實也是第四章裡面自我定義函式模式的一個例子。其缺點是，就如在第四章討
論過的，重新定義後的函式（在本例中是
Universe()
建構式）會失去在初次定義到重新
定義之間加在它身上的屬性。在我們現在具體的情況，你加在
Universe()
原型的屬性不
會及時連結（live link）到稍早用原本實作建立的實體。
以下的測試可以讓你看出問題：
[144
]
測試看看：
快取的實體
用一般的方式繼續
重新定義建構式
試試看
新增屬性至原型
再次新增屬性至原型
只有一開始的原型有連接到物件上
在建立初始物件之後
設計模式
149
之所以
uni.constructor
不再和
Universe()
建構式相等，是因為
uni.constructor
仍指
向著原本的建構式，而不是重新定義後的那一個。
若因為需求，要讓原型和建構式指
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.